The present day 40 G/100 G digital coherent communication employs modulation formats such as differential quadrature phase shift keying (DQPSK). To realize 400 G communication in the future, application of multi-level modulation formats such as quadrature amplitude modulation (16 QAM) is being studied for the next-generation modulation format.
One of the important characteristics of the light source required for multi-level modulation formats is a narrow line-width. One of the effective light sources for realizing a narrow line-width is an external resonator type variable wavelength laser. An external resonator type variable wavelength laser has a resonator configured with a semiconductor optical amplifier (SOA) and an external reflection mirror, selects a wavelength by a variable wavelength filter inserted into the resonator and outputs light of the wavelength. An external resonator type variable wavelength laser is disclosed, for example, in PTL 1-3.
FIG. 9 is a plan view of a variable wavelength laser device described in PTL 1. The variable wavelength laser device 900 according to PTL 1 includes a SOA 910, a variable wavelength unit 920, a phase-variable unit 930, a reflection mirror unit 940, a first optical waveguide 951, and a second optical waveguide 952.
The SOA 910 supplies light to the ring resonator type variable wavelength unit 920.
The variable wavelength unit 920 is a multiple light resonator formed by coupling by optical coupling means three ring resonators 921, 922, 923 having optical path lengths different from one another. The variable wavelength unit 920 controls the temperatures of the ring resonators 921, 922, 923 by using film heaters 924, 925, 926, respectively, to control the wavelength of light passing through the first optical waveguide 951.
The phase-variable unit 930 changes the temperature of the second optical waveguide 952, which connects the variable wavelength unit 920 and the reflection mirror unit 940, by using a heater 931 to control the phase of light passing through the second optical waveguide 952.
The reflection mirror unit 940 totally reflects the light passing through the variable wavelength unit 920 and the phase-variable unit 930 and returns the light to the phase-variable unit 930 and the variable wavelength unit 920.
In the variable wavelength laser device 900 configured as described above, resonant light is generated by the light outputted from the SOA 910, passing through the variable wavelength unit 920 and the phase-variable unit 930, and reflected by the reflection mirror unit 940. The SOA 910 only transmits light of a predetermined wavelength to accomplish single longitudinal mode oscillation.